Which nitrogen uptake form is considered immobile in the soil?

Ammonium is considered the immobile form of nitrogen in the soil due to its positive charge, which allows it to adhere tightly to soil particles. This electrostatic attraction reduces its likelihood of leaching or moving easily with soil water. As ammonium binds to the negatively charged surfaces of clay particles or organic matter, it remains localized in the root zone, making it readily available for plant uptake.

In contrast, other forms of nitrogen, such as nitrate, are negatively charged and tend to move freely with water through the soil, increasing their chances of leaching away from the root zone. Urea, while it can be hydrolyzed to ammonium in the soil, is initially less stable and needs to be converted before becoming available to plants. Nitrite is generally a transient state in the nitrogen cycle and is not a primary form of nitrogen uptake by plants. Understanding these characteristics helps in effectively managing nitrogen applications in agriculture to optimize plant health and reduce environmental impact.